1. Field of the Invention
This invention relates to the making of beads of expandable polystyrene, and in particular, to the making of relatively fine-particle beads of expandable polystyrene which has been modified in composition to improve flame resistance by making the beads with an effective proportion of a suitable organic bromide. Still more particularly, the invention concerns the making of beads of the kind indicated above in which hexabromocyclododecane, tetrabromocyclooctane or tetrabromovinylcyclohexane is used as the organic bromide in conjunction with certain peroxides, and in most instances, it concerns the making of such beads by a suspension polymerization process from, for the most part, monomeric material, although the process is applicable to the making of such fine-particled beads by a practice involving a preliminary formation of the polymer beads and then a subsequent treatment of them with other ingredients to bring them to the desired composition.
2. Description of the Prior Art
There has been, in connection with the usual practice of the prior art, a problem in connection with "shrinkage" and "collapse" of blocks molded from the anti-flame-modified expandable-polystyrene beads. The term "shrinkage" refers to the decrease in longitudinal or vertical dimension exhibited by a molded block upon being unmolded and permitted to cool to room temperature. The term "collapse" refers to the extent of in-bowing of a central portion of a side of such block. These blocks, which are usually thereafter intended to be cut into slab-like pieces 25 millimeters to 150 millimeters thick and used as insulation boards or the like, are relatively large. Generally, the blocks are on the order of 50 centimeters wide, 124 centimeters high, and 490 centimeters long. The percent of shrinkage can be determined by measuring the length of the product block at room temperature and comparing the dimension obtained with the known interior length dimension of the mold from which the block was produced. The percent of collapse can be determined by positioning a straightedge horizontally along one side of the block, about halfway up its height, and then measuring the distance between the straightedge and the in-bowed surface of the block. The same operation is repeated on the other side. Then, adding the two dimensions so obtained and comparing them with the nominal width of the block, as obtained from the known interior dimensions of the mold from which the block was produced, provides the percent of "collapse."
The collapse and shrinkage problems are not noticed to any particular degree in making of similar products from polystyrene compositions which have not been modified by the inclusion of organic bromide and peroxide synergist. As time goes on, however, the demand in the marketplace has increasingly been for the flame-retardant-modified material rather than the unmodified.
The problems of shrinkage and collapse have been known to be related to the bead size of the modified-composition expandable polystyrene used; there is in any event quite a bit of variability, there is somewhat less of a problem when the beads are larger. Thus, many producers prefer to work with the larger beads, because they provide a lesser degree of collapse and shrinkage. At the same time, however, the relatively larger beads give a product which is sometimes not considered as desirable for aesthetic reasons. Some producers find it necessary to accept the disadvantages (i.e., greater block collapse and shrinkage, and greater scrap losses in the slab-cutting operation) that go with making the desired product from fine-particle beads. The prior art has not indicated any way that these problems can be alleviated without substantially lengthening molding cycles.
Eichhorn et al, U.S. Pat. No. 3,058,928, relates to the field of making self-extinguishing expanded-plastic compositions. The reference teaches the use of organic bromide flame-proofing agents and an organic peroxide as a synergist. The reference requires that the organic peroxide be relatively stable, as indicated by a half-life of at least two hours at 100.degree. C., when tested in benzene. The reference teaches 1,1,2,2-tetrabromoethane as the organic bromide and dicumyl peroxide (DCP) as the peroxide synergist. The reference further teaches that the organic bromide is present at 0.2 to 5 percent by weight, preferably 0.5 to 2 percent by weight, and the peroxide synergist is present at 0.05 to 2 percent by weight, preferably from 0.2 to 1.5 percent by weight, based on the weight of the styrene.
Vanderhoff et al, U.S. Pat. No. 3,058,929, is similar in its teachings to that of the '928 patent.
Pillar et al, U.S. Pat. No. 3,819,547, is a patent in the art of making self-extinguishing expandable polystyrene, which teaches that such compositions are made by incorporating with the styrene 0.2 to 10 percent by weight of hexabromo-2-butene, with or without a peroxide synergist. The reference teaches as an organic peroxide synergist, 1,3-bis(alpha-tert-butylperoxyisopropyl)benzene, which is TBD. However, the reference is distinct herefrom in its teachings of the level of synergist employed. The patent contains no signpost to indicate that there is any advantage in using TBD in place of DCP.
Mueller-Tamm et al, U.S. Pat. No. 3,093,599, teaches making expandable polystyrene compositions which contain 0.5 to 6 percent by weight of organic polybromine compound, such as hexabromocyclododecane (HBCD). The problem of shrinkage or collapse is not discussed.
There is presently existing commercial practice wherein an organic bromide other than HBCD is used, and the K value of the polystyrene is over 60. This yields as a product, polystyrene beads which are relatively free of collapse and shrinkage, but this advantage is accompanied by drawbacks such as the need for increased mold-cycle times and the inconvenience of needing to add the bromide and/or peroxide synergist separately. There is, moreover, need for more storage and/or drying time and/or a slower throughput rate in the step of pre-expanding the beads before molding.
The prior art has not taught how the problem of reduced yield (because of shrinkage and/or collapse) could be alleviated without also accepting other drawbacks such as those mentioned above. In particular, the prior art has never suggested that, by the selection of a particular peroxide synergist, any such result could be obtained.